In Electronic Toll Collection (ETC) systems, Automatic Vehicle Identification (AVI) is achieved by the use of Radio Frequency (RF) communications between roadside readers and transponders within vehicles. Each reader emits a coded identification signal, and when a transponder enters into communication range and detects the reader, the transponder sends a response signal. The response signal contains transponder identification information, including a unique transponder ID. In the United States, current AVI RF communication systems are licensed under the category of Location and Monitoring Systems (LMS) through the provisions of the Code of Federal Regulations (CFR) Title 47 Part 90 Subpart M.
Current ETC systems can be classed as either lane-based or open-road. In a lane-based system, vehicles are laterally constrained by physical means, such as barriers between lanes, so as to prevent a vehicle from changing lanes while in the communication zone. The reader controls reader channels, each of which corresponds to RF coverage of an individual vehicle lane. In certain lane-based systems the capture zone is typically designed to be less than one car length in length, for example, approximately 2.4 meters (8 ft long) and 3 meters (10 feet) wide. Thus, when a vehicle with a transponder passes through a capture zone, the vehicle location is associated with the specific lane at that instant in time, and the short length of the zone allows for accurate timing alignment with the vehicle detection imaging systems.
In contrast, open-road systems allow traffic to free flow without impediment of lane barriers. Although many open-road systems have capture zones similar in size to those used in lane-based systems, the vehicles are not constrained to a particular lane. For example, they can be mid-way between two lanes, and need not be traveling parallel to the lanes. For example, a vehicle may be changing lanes as it passes through the toll area.
Furthermore, open-road systems may employ more channels than lanes to provide overlapping or staggered RF capture zones over multiple lanes. The reader analyses detections from multiple capture zones to determine to which zone to assign the vehicle location. This is sometimes referred to as a “voting” algorithm, since the capture zone that receives the most responses from a transponder indicates the corresponding vehicle's likely location. An example of such an ETC system is described in a commonly owned U.S. Pat. No. 6,219,613.
When an ETC system is first installed, whether it be lane based or open-road, RF-link margin tests are performed as part of what is referred to as a “lane tuning” process. Lane tuning aims to calibrate the RF power transmitted by the each antenna controlled by the reader. The RF link margin reflects the amount of additional RF attenuation that can be tolerated between the reader and a given transponder before communications become unreliable. In an ETC system, a balanced RF margin is desired for optimal performance. A too high RF margin may cause undesired “cross-lane” reads whereby a transponder is triggered in an adjacent lane, which may affect the localization accuracy. On the other hand, an RF margin that is too low results in unreliable and possibly no communication with some vehicle/transponder combinations. Therefore, a balanced RF margin is sought when first installing an ETC system.
Lane tuning typically includes the generation of a static RF margin map, whereby the RF margin is determined at multiple points within the capture zone. The RF margin can be determined by the use of a physical variable attenuator, or digitally controlled variable attenuator, whereby attenuation is increased up to the point at which communication between a reader antenna and a transponder mounted in a stationary vehicle ceases. If the ETC system is downlink limited (when there is greater transponder to reader uplink margin versus reader to transponder downlink margin), as is assumed in the following description, a common attenuator that applies to both transmit and receive paths measures the downlink margin. Lane tuning may also involve determining a dynamic peak RF margin, whereby the maximum RF margin is recorded as a vehicle drives through a capture zone multiple times. The process requires an operator to be on-site with a test vehicle and a reference transponder, and requires the lane under test to be closed to traffic.
Over time, the RF margin can change (it typically decreases due to component degradation, weather, or other factors), which negatively impacts the communications link between the reader and transponder. One option is to periodically re-test the lane tuning by repeating all or part of the activities performed when a lane is initially commissioned. For example, a dynamic margin test can be re-performed; however, this requires that the corresponding lane be closed to traffic for the duration of the test, which under current procedures can take hours to complete.
Some ETC systems may employ Received Signal Strength Indication (RSSI) in the receiver block of a reader RF module in order to estimate RF link margin. The present invention describes a method which is not dependent on measuring received signal strength at the reader.
It would be advantageous to provide for improved processes and systems for RF-link margin testing in an ETC system, especially one suited to vehicles travelling at highway speeds.